Remotely sensing optical tachometer

ABSTRACT

A remotely sensing optical tachometer for measuring the spin rate of a rotating member. The invention provides a simple optical system for measuring the spin and/or rate of change of spin, of, for example, an artillery shell or test projectile. The system provides a real-time readout of the spin rate, and requires no instrumentation aboard the projectile except for a small plane mirror that is attached obliquely to one end of the spinning projectile. The system directs a collimated beam of light that is reflected off the plane mirror back into the system where it traverses a stationary reticle that has alternating transparent and opaque sectors thereon. The frequency with which the light passes through the transparent portions of the reticle provides a direct indication of the spin rate of the rotating member. The signal may be detected by a photo tube, amplified, and displayed on an oscilloscope, or recorded on magnetic tape, or be processed through a frequency-to-DC converter.

United States Patent Mary et al.

[ 51 Apr. 4, 1972 [54] REMOTELY SENSING OPTICAL TACHOMETER [72]Inventors: Donald J. Mary, l-lyattsville; Harry J.

Davis, Wheaton, both of Md.

[73] Assignee: The United States of America as represented by theSecretary of the Army 22 Filed: July 1,1910

21 Appl.No.: 51,461

[52] US. Cl ..324/175, 73/167, 250/230,

OTHER PUBLICATIONS F. B. Porterfield, Photoelectric Circuit Breaker,"IBM Tech. Discl. BuL, 1 (Dec. 1958) p. 8.

Electronic Tachometer," Electronic Industries, Vol. 4, No. 3, Mar. 1945,pp. 80- 81, 208.

Primary ExaminerRonald L. Wibert Assistant ExaminerJeff RothenbergAtt0rney--Harry M. Saragovitz, Edward J. Kelly, Herbert Her] and J. D.Edgerton [5 7] ABSTRACT A remotely sensing optical tachometer formeasuring the spin rate of a rotating member. The invention provides asimple optical system for measuring the spin and/or rate of change ofspin, of, for example, an artillery shell or test projectile. The systemprovides a real-time readout of the spin rate, and requires noinstrumentation aboard the projectile except for a small plane mirrorthat is attached obliquely to one end of the spinning projectile. Thesystem directs a collimated beam of light that is reflected off theplane mirror back into the system where it traverses a stationaryreticle that has alternating transparent and opaque sectors thereon. Thefrequency with which the light passes through the transparent portionsof the reticle provides a direct indication of the spin rate of therotating member, The signal may be detected by a photo tube, amplified,and displayed on an oscilloscope, or recorded on magnetic tape, or beprocessed through a frequency-to-DC converter.

12 Claims, 2 Drawing Figures Patented April 4, 1972 AMPL\F\ER *I I l 1o19 I EXT. OSCILLOSCOPE 'UGGER l g I FREQUENCY I TO 0c CONVERTER/M/EA/7'O/Z5, 00/V4L0 J. Ma/zr REMOTELY SENSING OPTICAL TACHOMETERRIGHTS OF GOVERNMENT The invention described herein may be manufactured,used, and licensed by or for the Government of the United States forgovernmental purposes without the payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION I artillery shells or other spinningprojectiles, knowledge of the spin rate and spin rate of change of theprojectile is important.

Present techniques for determining the spin rate of such a projectileare photographing the projectile with a high-speed movie camera ortelemetering the output of accelerometers mounted in the projectile. Theformer method requires considerable time to reduce the data and involvesexpensive photography equipment. The latter method can also be highlyexpensive in that the telemetry unit may be destroyed when theprojectile impacts.

Accordingly, the primary object of the present invention is to providean optical device that will remotely sense the spin rate and spin rateof change of a rotating projectile in which no instrumentation isrequired on the projectile.

Another object is to provide an optical tachometer for measuring thespin and rate of change of spin of a test projectile that allowsreal-time readout of the spin rate.

An additional object of the present invention is to provide a simpleoptical system which allows a remote measurement of rotation of aspinning member that is inexpensive and simple to construct.

SUMMARY OF THE INVENTION Briefly, in accordance with the invention, aremotely sensing optical tachometer is provided that allows real-timereadout of the spin rate and/or spin rate of change of a rotatingprojectile that is remote to the measuring system. The tachometercomprises a plane mirror that is mounted obliquely along the spin axisof one end of the rotating member and an optical system for directingand detecting a collimated beam of light onto said mirror. Since themirror is mounted at an angle to its spin axis, the reflected lightproperly focused traverses a circular path due to the rotation of theprojectile. A stationary reticle having alternating transparent andopaque sectors is placed in the optical path of the reflected beam. Thecircular path traversed by the reflected beam upon the reticle providesa series of pulses to a photo tube placed behind the reticle. The seriesof pulses will generate a signal whose frequency will be directlyproportional to the spin rate of the projectile. By making the number ofsectors in the reticle sufficiently large, the rate of change of spinmay be detected over very small time intervals. The output of the phototube may be amplified and fed to either an oscilloscope or to afrequencyto-DC converter to provide a real-time plot of spin rate versustime.

BRIEF DESCRIPTION OF THE DRAWINGS The specific nature of the inventionas well as other objects, aspects, uses, and advantages thereof willclearly appear from the following description and from the accompanyingdrawings, in which:

FIG. 1 is a schematic illustration of a preferred embodiment of theoptical tachometer of the present invention; and

FIG. 2 is a drawing of a reticle which is employed in the preferredembodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The device of the presentinvention provides simple optical means for measuring the spin and/orrate of change of spin of a projectile. The method provides real-timereadout of the spin rate, and requires no instrumentation aboard theprojec tile except a small plain mirror. The technique is ideally suitedfor use with a device such as disclosed in US. Pat. No. 3,444,733 toCurchak for an artillery simulator. However, this invention may be usedwith other types of air or gas guns, spinners, and centrifuges, orwherever the remote measurement of rotation is desired.

Referring now to FIG. 1, a spinning projectile 6 is shown moving awayfrom the apparatus along its spin axis 11. A tilted plane mirror 7 ismounted on the rear end of projectile 6. A light source 1 is imaged by apair of condensing lenses 2 onto a pinhole 3. Pinhole 3 is located inthe focal plane of a lens 5 that is centered on spin axis 11 ofprojectile 6. Between lens 5 and and pinhole 3 is a beam splitter orhalf-silvered mirror 4 which directs the light from pinhole 3 to lens 5.Since pinhole 3 is located in the focal plane of lens 5, the lightemerges from lens 5 in a collimated beam represented by rays 10. If thesurface of mirror 7 mounted upon projectile 6 is tilted at an angle 0from the normal to spin axis 11, some of the light in beam 10 will bereflected, as represented by reflected beam 12, at an angle 20 Becausereflected beam 12 is also collimated, an image 13 of source 1 will beformed in the focal plane of lens 5. Although some of the light in beam12 will be reflected down to source 1 by half-silvered mirror 4, aportion will still pass through and be imaged at 13. A stationaryreticle 14 is located in the focal plane of lens 5 and the light formingimage 13 impinges upon it.

As seen in FIG. 2, reticle 14 consists of a series of alternatetransparent 24 and opaque 22 sectors equally spaced around itscircumference. If projectile 6 is rotating about axis 11 while inflight, reflected beam 12 will also rotate about spin axis 11. It isevident that image 13 will likewise rotate about reticle 14 in a circleof radius r. Behind reticle 14 is a photoelectric tube 17 or some othersuitable photosensitive detector. As image 13 rotates about reticle 14,light passes through to phototube 17 each time the image moves over atransparent sector of reticle 14. The light is blocked from phototube 17when image 13 passes over an opaque sector of reticle 14. Thus, theoutput of photoelectric tube 17 will be a signal whose frequency is adirect function of the spin rate of projectile 6. The output ofphototube 17 is amplified by amplifier 18 and displayed on anoscilloscope 19 where it may be viewed or photographed. Oscilloscope 19may be triggered from some external source 20 so that only a smallportion of the flight path of the projectile is displayed but on amagnified time scale. Additionally, the amplified signal from amplifier18 may be recorded on magnetic tape, or processed through afrequency-to-DC converter 21. The output of the converter 21 whendisplayed on an oscilloscope 19 will produce a real-time plot of spinrate versus time. Between reticle 14 and phototube 17 is a field lens 15which serves to direct the light onto phototube 17 regardless of whereon reticle 14 image 13 is formed.

Any collimated source of light may be used to generate beam 10. Forexample, a small helium-neon gas laser could be utilized. Its use wouldeliminate the necessity for condensing lenses 2 and pinhole 3. In suchan arrangement beam splitter 4 can be reduced in size as it need only beas large as the diameter of the laser beam. In instances whereprojectile 6 is fired from a gun which is closed at the rear end by abreach or some other assembly, head on measurement of the spin ispossible by using a plane mirror mounted on the front of the projectile.

We wish it to be understood that we do not desire to be limited to theexact details of construction shown and described for obviousmodifications will occur to a person skilled in the art.

We claim as our invention:

1. An optical tachometer for measuring the spin rate of a rotatingmember along its trajectory comprising:

a. a rotating member having a spin axis oriented substantially along itstrajectory;

b. a reflecting surface mounted obliquely with respect to said spin axison said rotating member;

c. means for directing a beam of light towards said reflecting surface,said beam being reflected from said reflecting surface, said reflectedbeam rotating in space by virtue of the spin associated with saidrotating member;

d. means remotely located from said rotating member for receiving saidrotating and reflected beam of light; and

e. means for measuring the rate of rotation of said reflected beam oflight as said rotating member travels along said trajectory and as theforward motion of said rotating member ceases whereby the spin rate ofsaid rotating member may be readily ascertained at any point along saidtrajectory.

2. The invention according to claim 1 wherein said measuring meanscomprises:

a. a reticle having a plurality of alternating transparent and opaquesectors whereon said reflected beam of light traverses a predefinedpath; and

b. means located behind said reticle for detecting the pulses of saidreflected beam of light allowed therethrough and for producing an outputsignal whose frequency is proportional to the spin rate of said rotatingmember.

3. The invention according to claim 2 wherein said means for directing abeam of light towards said reflecting surface comprises:

a a source of light;

b. condensing lenses for receiving the light from said source;

c. means for providing a pinhole for receiving the light from saidcondensing lenses and transmitting a portion therefrom; and

d. means located along said spin axis of said rotating member forreflecting said portion of light from said means for providing a pinholetowards said rotating member.

4. The invention according to claim 3 wherein said receiv ing meanscomprises a lens centered on said spin axis of said rotating member.

5. The invention according to claim 4 wherein said reticle is located inthe focal plane of said lens and perpendicular to said spin axis andwherein said transparent and opaque sectors are radially disposedthereupon.

6. The invention according to claim 5 wherein said reticle is stationaryrelative to said rotating member.

7. The invention according to claim 6 wherein said reflecting meanscomprises a beam splitter.

8. The invention according to claim 6 wherein said reflecting meanscomprises a half-silvered mirror.

9. The invention according to claim 2 wherein said detecting meanscomprises a photosensitive device that produces said output signal,further comprising an amplifier and means for displaying the amplifiedsignal therefrom.

10. The invention according to claim 9 further comprising afrequency-to-DC converter that receives said amplified signal andproduces a real-time plot of said spin rate versus time.

1 1. The invention according to claim 9 further comprising a field lenslocated between said reticle and said photosensitive device fordirecting said reflected light onto said photosensitive deviceregardless of its location upon said reticle.

12. The invention according to claim 11 wherein said reflecting surfacecomprises a plane mirror.

1. An optical tachometer for measuring the spin rate of a rotatingmember along its trajectory comprising: a. a rotating member having aspin axis oriented substantially along its trajectory; b. a reflectingsurface mounted obliquely with respect to said spin axis on saidrotating member; c. means for directing a beam of light towards saidreflecting surface, said beam being reflected from said reflectingsurface, said reflected beam rotating in space by virtue of the spinassociated with said rotating member; d. means remotely located fromsaid rotating member for receiving said rotating and reflected beam oflight; and e. means for measuring the rate of rotation of said reflectedbeam of light as said rotating member travels along said trajectory andas the forward motion of said rotating member ceases whereby the spinrate of said rotating member may be readily ascertained at any pointalong said trajectory.
 2. The invention according to claim 1 whereinsaid measuring means comprises: a. a reticle having a plurality ofalternating transparent and opaque sectors whereon said reflected beamof light traverses a predefined path; and b. means located behind saidreticle for detecting the pulses of said reflected beam of light allowedtherethrough and for producing an output signal whose frequency isproportional to the spin rate of said rotating member.
 3. The inventionaccording to claim 2 wherein said means for directing a beam of lighttowards said reflecting surface comprises: a a source of light; b.condensing lenses for receiving the light from said source; c. means forproviding a pinhole for receiving the light from said condensing lensesand transmitting a portion therefrom; and d. means located along saidspin axis of said rotating member for reflecting said portion of lightfrom said means for providing a pinhole towards said rotating member. 4.The invention according to claim 3 wherein said receiving meanscomprises a lens centered on said spin axis of said rotating member. 5.The invention according to claim 4 wherein said reticle is located inthe focal plane of said lens and perpendicular to said spin axis andwherein said transparent and opaque sectors are radially disposedthereupon.
 6. The iNvention according to claim 5 wherein said reticle isstationary relative to said rotating member.
 7. The invention accordingto claim 6 wherein said reflecting means comprises a beam splitter. 8.The invention according to claim 6 wherein said reflecting meanscomprises a half-silvered mirror.
 9. The invention according to claim 2wherein said detecting means comprises a photosensitive device thatproduces said output signal, further comprising an amplifier and meansfor displaying the amplified signal therefrom.
 10. The inventionaccording to claim 9 further comprising a frequency-to-DC converter thatreceives said amplified signal and produces a real-time plot of saidspin rate versus time.
 11. The invention according to claim 9 furthercomprising a field lens located between said reticle and saidphotosensitive device for directing said reflected light onto saidphotosensitive device regardless of its location upon said reticle. 12.The invention according to claim 11 wherein said reflecting surfacecomprises a plane mirror.